a. Field of the Invention
The present invention relates to thin film coatings and to physical vapor deposition methods for manufacturing such coatings. More specifically, our invention also relates to protective thin film optical coatings formed on, and/or which include, optical detectors and to methods of manufacturing such coatings. Still more specifically, our invention also relates to solar cells, protective thin film optical coatings for such cells, and to methods of forming such coatings.
As used here in reference to our invention, the term "optical" coating (or cover) means a coating which affects reflection, transmission, absorption or scatter of electromagnetic energy at an interface.
b. Current state of the Relevant Technology
(1) Overview of Solar Cell Construction
A typical solar cell is depicted schematically in FIG. 1. (The illustrated cell 10 is not drawn to scale; relative dimensions are altered for ease of viewing. Also, various conventional components, such as the electrical connections to associated cells in an array, are omitted for clarity.) The typical cell 10 comprises a substrate 11 of semiconductor material (single crystal silicon, polycrystalline silicon (polysilicon), amorphous silicon, GaAs (gallium arsenide),etc.) having conductors 13 formed on its front surface and associated, surface-adjacent pn junctions (not shown) for converting solar radiation of selected wavelength range into electrical current and power. The electrical current is conducted by a bus bar 14 connected to the front side conductors 13 and by conductor coating 12 formed on the back of the substrate to other cells in the array and ultimately to devices which are operated by the cell(s) and/or to storage batteries. Anti-reflection (AR) coating 15 is formed on the front of the substrate 11. The substrate 11 is covered by a protective glass cover 18, which is attached to the substrate (or, as here, to the AR coating on the front of the substrate) by adhesive layer 16. The AR coating 15 is optically matched to the adhesive layer 16.
Optionally, a so-called "blue", ultra-violet (UV) reflector coating 17 can be formed on the rear surface of cover glass 18, to protect the adhesive from UV radiation. Optional layer(s) 19 formed on the outer front surface of the cover 18 comprise simple AR coatings of material such as magnesium fluoride; UV reflector coatings; conductive coatings for bleeding off static electricity, etc.; or combinations of such coatings.
Typical thickness values for these coatings are: simple AR, a fraction of a micron, e.g., 0.1 micron; blue UV reflector, less than or equal to one micron; and adhesive, 0.002 to 0.003 inch (50 to 75 microns). Suitable cover materials include various amorphous glasses and fused silica. Typical thickness ranges for the glass and the fused silica are about 0.003 in. to about 0.012 in. and about 0.005 in. to about 0.040 in., respectively. The useful thickness range is determined by the requirements that the cover be able to withstand handling (the covers and cells are handled repeatedly during the coating and sizing processes), by emissivity, and by the need for radiation hardness. In particular, the very thick covers are for cells on spacecraft which traverse the Van Allen belt.
(2) Solar Cell Design Considerations
Our consideration here is primarily solar cells used in space applications, that is, those carried on spacecraft such as satellites, shuttles and space stations, with emphasis on the critical cover and adhesive technologies. Such applications require that, first, the cover have excellent integrity, characterized for example by the absence of pinholes that render the cell subject to attack by the environment in space. Second, the cover, adhesive and other components are selected and constructed to be of the lightest possible weight, consistent with the other, often conflicting, stringent requirements for the solar cell. The other requirements include the dual requirements that the cover provide sufficient radiation hardness and have high thermal emissivity. Fifth, the cover must have high optical transmissivity over the radiation wavelength range of interest. Our primary concern here is the short wavelength portion of the solar emissions spectrum from about 280 nm (nanometer) to about 700-1100 nm. High transmissivity over this range ensures that the high energy, short wavelength radiation is available for conversion to electrical energy at the pn junctions and is not wasted in generating heat. Sixth, the cell should not be degraded by the fabrication process, as measured, e.g., by electrical parameters such as open collector voltage, short circuit current, and the fill factor. Two additional, closely related requirements are the ability of the cell to withstand thermal cycling between temperature extremes, as indicated by the standard temperature test cycle of -196.degree. C. to +175.degree. C.; and the requirement that the cover and the substrate have closely matched thermal expansion coefficients, to avoid warpage and breakage during the temperature excursions experienced by the cell.
Typically, the adhesive 16 which is used to mount and seal the glass cover 18 to the substrate 11 is the most critical component affecting cell integrity and thermal durability in that it is the most susceptible to deterioration at high temperatures and by UV radiation. Accordingly, it is desirable to eliminate the use of the adhesive bonded glass cover and, instead, to use a thin film coating as the cover. Unfortunately, in the past, the inability of thin film coatings to satisfy the above-discussed combination of objectives has precluded the use of such coatings as solar cell covers. Specifically, thin film technology has not been successfully used in this application because of the problems of (1) inadequate stress control and excessive stress in the very thick thin films which are required (for example, for adequate radiation hardness thicknesses of at least 10-50 microns may be required); (2) inadequate adhesion to the substrate; (3) non-uniform films; and (4) lack of scalability. Such relatively thick, thin films are characterized by uncontrolled, unacceptably high stress and associated warpage and breakage. The problem is not limited to space applications; in general, stress effects limit the useful thicknesses of optical thin films.